The squid giant synapse has served as a model for the understanding of the physiology of synaptic transmission but the transmitter acting at this synapse has not as yet been identified. The lack of pharmacological studies on this preparation is due primarily to the high diffusion barrier between the synapse and the bathing medium. In this study we are taking advantage of a novel method of pharmacological access to the synapse by arterial perfusion to identify the transmitter substance. First, we analyze, by high pressure liquid chromatography, chemicals released from the axon terminal into the perfusate during direct depolarization. Second, we compare the pharmacological action on the postsynaptic giant axon of putative transmitter substances identified in the perfusate with those of the endogenous transmitter. In addition, we are using arterial perfusion to examine the cAMP or serotonin-induced enhancement of transmitter release from the presynaptic axon terminal and the changes in ionic currents associated with this enhancement are explored by the voltage clamp technique.